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  MySQL事务隔离级别及锁机制
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<h2 id="Mysql事务隔离级别与锁机制"><a href="#Mysql事务隔离级别与锁机制" class="headerlink" title="Mysql事务隔离级别与锁机制"></a>Mysql事务隔离级别与锁机制</h2><h3 id="1、事务并发如何解决"><a href="#1、事务并发如何解决" class="headerlink" title="1、事务并发如何解决"></a>1、事务并发如何解决</h3><ul>
<li><strong>事务隔离机制</strong></li>
<li><strong>锁机制</strong></li>
<li><strong>MVCC多版本并发控制隔离机制</strong></li>
</ul>
<h3 id="2、事务及其ACID属性"><a href="#2、事务及其ACID属性" class="headerlink" title="2、事务及其ACID属性"></a>2、<strong>事务及其ACID属性</strong></h3><p>事务具有以下4个属性（ACID）</p>
<ul>
<li>原子性(Atomicity) ：事务是一个原子操作单元,其对数据的修改,要么全都执行,要么全都不执行。  从操作的维度</li>
<li>一致性(Consistent) ：在事务开始和完成时,数据都必须保持一致状态。这意味着所有相关的数据规则都必须应用于事务的修改,以保持数据的完整性。  从数据一致性的维度</li>
<li>隔离性(Isolation) ：数据库系统提供一定的隔离机制,保证事务在不受外部并发操作影响的“独立”环境执行。这意味着事务处理过程中的中间状态对外部是不可见的,反之亦然。</li>
<li>持久性(Durable) ：事务完成之后,它对于数据的修改是永久性的,即使出现系统故障也能够保持。</li>
</ul>
<h3 id="3、并发事务带来的问题"><a href="#3、并发事务带来的问题" class="headerlink" title="3、并发事务带来的问题"></a>3、并发事务带来的问题</h3><ul>
<li><p>更新丢失(Lost Update)或脏写</p>
<p>当两个或多个事务选择同一行，然后基于最初选定的值更新该行时，由于每个事务都不知道其他事务的存在，就会发生丢失更新问题–<strong>最后的更新覆盖了由其他事务所做的更新</strong>。</p>
</li>
<li><p>脏读（Dirty Reads）</p>
<p>一个事务正在对一条记录做修改，在这个事务完成并提交前，这条记录的数据就处于不一致的状态；这时，另一个事务也来读取同一条记录，如果不加控制，第二个事务读取了这些“脏”数据，并据此作进一步的处理，就会产生未提交的数据依赖关系。这种现象被形象的叫做“脏读”。</p>
</li>
</ul>
<p>　　一句话：<strong>事务A读取到了事务B已经修改但尚未提交的数据</strong>，还在这个数据基础上做了操作。此时，如果B事务回滚，A读取的数据无效，不符合一致性要求。</p>
<ul>
<li><p>不可重读（Non-Repeatable Reads） </p>
<p>一个事务在读取某些数据后的某个时间，再次读取以前读过的数据，却发现其读出的数据已经发生了改变、或某些记录已经被删除了！这种现象就叫做“不可重复读”。</p>
</li>
</ul>
<p>　　一句话：<strong>事务A内部的相同查询语句在不同时刻读出的结果不一致，不符合隔离性</strong></p>
<ul>
<li>幻读（Phantom Reads）</li>
</ul>
<p>　　一个事务按相同的查询条件重新读取以前检索过的数据，却发现其他事务插入了满足其查询条件的新数据，这种现象就称为“幻读”。</p>
<p>　　一句话：<strong>事务A读取到了事务B提交的新增数据，不符合隔离性</strong></p>
<h3 id="4、事务隔离级别"><a href="#4、事务隔离级别" class="headerlink" title="4、事务隔离级别"></a>4、事务隔离级别</h3><figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">常看当前数据库的事务隔离级别: show variables like &#x27;tx_isolation&#x27;;</span><br><span class="line">设置事务隔离级别：set tx_isolation=&#x27;REPEATABLE-READ&#x27;;</span><br><span class="line">Mysql默认的事务隔离级别是可重复读，用Spring开发程序时，如果不设置隔离级别默认用Mysql设置的隔离级别，如果Spring设置了就用已经设置的隔离级别</span><br></pre></td></tr></table></figure>



<p><img src="/2022/05/26/00-02-05-MySQL%E4%BA%8B%E5%8A%A1%E9%9A%94%E7%A6%BB%E7%BA%A7%E5%88%AB%E5%8F%8A%E9%94%81%E6%9C%BA%E5%88%B6/image-20220527000218134.png" alt="image-20220527000218134"></p>
<p>注：隔离级别控制的越严格，事务的副作用越小，性能会更低</p>
<h3 id="5、MySQL中的锁"><a href="#5、MySQL中的锁" class="headerlink" title="5、MySQL中的锁"></a>5、MySQL中的锁</h3><p>锁是计算机协调多个进程或线程并发访问某一资源的机制。</p>
<p><strong>锁分类</strong></p>
<ul>
<li>从性能上分为乐观锁(用版本对比来实现)和悲观锁</li>
<li>从对数据库操作的类型分，分为读锁和写锁(都属于悲观锁)</li>
</ul>
<p>读锁（共享锁，S锁(<strong>S</strong>hared)）：针对同一份数据，多个读操作可以同时进行而不会互相影响</p>
<p>写锁（排它锁，X锁(e<strong>X</strong>clusive)）：当前写操作没有完成前，它会阻断其他写锁和读锁</p>
<ul>
<li>从对数据操作的粒度分，分为表锁和行锁</li>
</ul>
<p><strong>表锁</strong>：一般用于迁移数据</p>
<p>INNER/MYISAM 存储引擎都有表锁</p>
<p>每次操作锁住整张表。开销小，加锁快；不会出现死锁；锁定粒度大，发生锁冲突的概率最高，并发度最低；一般用在整表数据迁移的场景。</p>
<p>不允许更新和插入</p>
<p><strong>行锁</strong></p>
<p>每次操作锁住一行数据。开销大，加锁慢；会出现死锁；锁定粒度最小，发生锁冲突的概率最低，并发度最高。</p>
<p>InnoDB与MYISAM的最大不同有两点：</p>
<ul>
<li><strong>InnoDB支持事务（TRANSACTION）</strong></li>
<li><strong>InnoDB支持行级锁</strong></li>
<li><strong>MYISAM不支持行锁，不支持事务</strong></li>
</ul>
<p><strong>锁总结</strong></p>
<p>MyISAM在执行查询语句SELECT前，会自动给涉及的所有表加读锁,在执行update、insert、delete操作会自动给涉及的表加写锁。</p>
<p>InnoDB在执行查询语句SELECT时(非串行隔离级别)，不会加锁。但是update、insert、delete操作会加行锁。</p>
<p>简而言之，就是<strong>读锁会阻塞写，但是不会阻塞读。而写锁则会把读和写都阻塞</strong>。</p>
<p><strong>间隙锁(Gap Lock)</strong></p>
<p>间隙锁，锁的就是两个值之间的空隙。在一定程度上可以解决幻读问题。</p>
<p>假设account表里数据如下：</p>
<p>​    <img src="/2022/05/26/00-02-05-MySQL%E4%BA%8B%E5%8A%A1%E9%9A%94%E7%A6%BB%E7%BA%A7%E5%88%AB%E5%8F%8A%E9%94%81%E6%9C%BA%E5%88%B6/image-20220527000626327.png" alt="image-20220527000626327"></p>
<p>那么间隙就有 id 为 (3,10)，(10,20)，(20,正无穷) 这三个区间，</p>
<p>在Session_1下面执行 update account set name = ‘zhuge’ where id &gt; 8 and id &lt;18;，则其他Session没法在这个<strong>范围所包含的所有行记录(包括间隙行记录)以及行记录所在的间隙</strong>里插入或修改任何数据，即id在(3,20]区间都无法修改数据，注意最后那个20也是包含在内的。【只要sql语句范围有一个落在了上面三个区间里，则那个区间就会被加锁】</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">举例说明：</span><br><span class="line">以上图作为例子</span><br><span class="line">Session_1执行update account set name = &#x27;zhuge&#x27; where id &gt; 8 and id &lt;18 事务还未提交</span><br><span class="line">Session_2执行了</span><br><span class="line">insert into account(id,name,balance)values(21,&#x27;xg&#x27;,123)  可以插入</span><br><span class="line">insert into account(id,name,balance)values(19,&#x27;xg&#x27;,123)  不可以插入 虽然你锁的范围是8~18 但实际占两个空间 即（3，10）、（10，20）</span><br><span class="line">**间隙锁是在可重复读隔离级别下才会生效。**</span><br></pre></td></tr></table></figure>



<p>**临键锁(Next-key Locks)**：间隙锁+表锁 = 临键锁</p>
<p>Next-Key Locks是行锁与间隙锁的组合。像上面那个例子里的这个(3,20]的整个区间可以叫做临键锁。</p>
<p><strong>无索引行锁会升级为表锁(RR级别会升级为表锁，RC级别不会升级为表锁)</strong></p>
<p>锁主要是加在索引上，如果对非索引字段更新，行锁可能会变表锁</p>
<p>session1 执行：update account set balance = 800 where name = ‘lilei’;（name没有加索引）</p>
<p>session2 对该表任一行操作都会阻塞住</p>
<p><strong>InnoDB的行锁是针对索引加的锁，不是针对记录加的锁。并且该索引不能失效，否则都会从行锁升级为表锁****。</strong></p>
<p>锁定某一行还可以用lock in share mode(共享锁) 和for update(排它锁)，例如：select * from test_innodb_lock where a = 2 for update; 这样其他session只能读这行数据，修改则会被阻塞，直到锁定行的session提交</p>
<p><strong>锁优化建议</strong></p>
<ul>
<li>尽可能让所有数据检索都通过索引来完成，避免无索引行锁升级为表锁</li>
<li>合理设计索引，尽量缩小锁的范围</li>
<li>尽可能减少检索条件范围，避免间隙锁</li>
<li>尽量控制事务大小，减少锁定资源量和时间长度，涉及事务加锁的sql尽量放在事务最后执行</li>
<li>尽可能低级别事务隔离</li>
</ul>
 
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